I would definitely not go for the Corsair XMS if I was you. True, it is extremely fast, however, I doubt you will notice anything over what you have. It is more of overclocker's memory, if that is what you want to do. For me this is an easy decision:
2x512 of Corsair XMS Low Latency: $270
1x512 stick of current Kingston ValueRAM: $80
That isn't a hard decision. The difference in speed is so small that it in NO WAY justifies spending an extra $190 dollars.
As for getting 3 GB of RAM to max it out, I say no as well. Unless you're doing video editing or heavy CAD work, you don't need any more than 1 GB at all. As we speak, I have open an anti-virus program, spy sweeper, Winamp(with music playing), 2 IE windows, and a chess program and am experience no slowdown with 512 MB of RAM. I couldn't think of any reason you would need more than 1 GB.
Regarding your video card, keep what you have. Right now, that is one of the best video cards available. See for yourself here, to see how it compares with others in Doom 3: http://graphics.tomshardware.com/gra...809/index.html
As far as the hard drive goes, if you're doing fine with 80 GB, stick with it. If you're running out of space, then get another 120 or 160 GB. A 74 GB raptor costs about $160-180 dollars, which is A LOT of money for some speed. Once again, I don't think it is worth the upgrade for that much money.
Of course it's your decision, but if I was you, I would do the following:
(1) Get another 512 MB stick of Kingston ValueRAM for a total of 1 GB
(2) If you need more hard drive space, get another one that is 120 or 160 GB. If you're doing fine with what you have now, then don't change anything.
(3) Keep the video card. It is a great one already, there's no need to upgrade now to a 6800 GT/Ultra.
In conclusion, if you just upgrade your RAM, that is only $80 bucks, which still leaves you with $220 to put towards future upgrades. You could save up for a 6800 GT/Ultra. That's just my input, but do what you feel like.